The use of LED bulbs to replace incandescent bulbs in 12V applications is gaining popularity in a large number of applications such as head lights in automobiles and other motor vehicles, as well as for indoor lighting within the home, work place, boats and recreational vehicles. Incandescent bulbs, including most halogen bulbs, consume a large amount of power, generate a large amount of heat and have a relatively short life. They provide only a warm white illumination. Conversely, LED bulbs consume less power, generate less heat and have a longer operating life, although they tend to be more expensive than incandescent bulbs.
For many applications, it is desirable to be able to vary the emitted light intensity of a LED bulb, for example between full brightness (bright) and a less bright (dim) intensity. One way of doing this is by decreasing the forward current using variable resistors, voltage regulators, or LED drivers, for example.
However, LEDs have a minimum voltage threshold at which the LED illuminates. They also have a range of safe current operation, above which they will overheat and burn. These thresholds can vary between individual LEDs due to manufacturing differences. These differences are not usually visible at the recommended forward driving current but at lower currents some of the LEDs can stop illuminating entirely. Also, due to slight colour differences between LEDs and the LED emitted light intensity not being linear with current, the colour spectrum can shift at currents below full rating, which can result in some LEDs appearing a different colour. These unpredictable and non-uniform lighting effects are undesirable and particularly inappropriate for some applications such as vehicle head lights where a uniform appearance is required for safety. Also, as the emitted light intensity is not linear with current, the dimming effects are unpredictable and uncontrollable.
For LED light bulbs that use a resistor, such as a shunt resistor, to regulate the operating current in LED strings, the relationship between the emitted light intensity and current is more linear than with LED bulbs including drivers and regulators. However, these bulbs are designed to operate at a fixed voltage, typically 12V. When the operating voltage varies or is not stable, which is often the case in automotive applications, the emitted light intensity varies accordingly. Also, these bulbs still have a minimum voltage threshold below which they cannot light up (which is under 8V for 3 LEDs connected in series) and they burn-out when the input voltage is higher than the one for which it is designed. Therefore, these are not suitable for use in automotive applications such as in Recreational Vehicles where the voltage typically ranges from 6V to 16V (with 12V as typical voltage).
Another approach to varying the emitted light intensity of LED bulbs is pulse width modulation (PWM) which involves turning the LEDs on and off at a high frequency so that the human eye cannot see the strobing effect. The longer the ‘on’ period relative to the ‘off’ period of the LED, the brighter the LED will appear. However, the LED circuits are complex and require a timer or an oscillator to control the on/off, which adds to the expense. Also, switching speeds need to exceed the response time of the human eye. Therefore, the power supply driving the LED must also operate at high frequencies. Finally, this kind of device tends to generate audible noises.
Therefore, there is a need for an improved LED bulb which overcomes or reduces at least some of the above described problems.